Trucks, as well as other large vehicles, generally use air brakes for stopping for both the tractor and the trailer. Air brakes include a service brake chamber which drives a push rod into a brake on position when air is applied to the service brake chamber. For parking purposes, a dual chamber air brake is often used. These dual chambered brakes include both a service brake chamber, for applying the brakes during normal operation, and what is called a spring brake chamber. The spring brake chamber includes a relatively heavy spring connected to the push rod. The spring is compressed only when air from a spring brake line is applied to the spring brake chamber. When the air in the spring brake chamber is vented, the spring brake spring drives the push rod to a brake on position. Thus, when a truck is parked and the air in the brake chambers is vented, the service brake chamber no longer actuates the brakes but the spring brake spring pushes the push rod into a spring brake on position thus applying the brakes.
Conventional dual chamber brakes can only be made so that the braking force applied by the spring brake spring is only about half of the braking force applied by the pressurization of the service brake chamber. This has made the use of the legal parking brake inadequate for emergency braking requirements, and has limited its effective usage to parking brake requirements only. The rationale for this seemingly defective design is that the spring brake chamber may be vented while the service brake chamber is pressurized; when this occurs the total force on the brake is greater than that of the service brake alone. If the spring brake spring were made large enough to equal the force resulting from the pressurization of the service brake chamber, the force applied to the brakes would be about double that applied by the service brakes alone. This excess loading may lead to cracked drums and other premature failure. Thus the spring brake force is necessarily and commonly limited to about 50% of the service brake force. Recently there are means available to prevent the compounding of the service and parking brakes forces. See U.S. Pat. No 4,407,548.
At present, there are no statutes which provide for emergency brakes for air braked vehicles. There are statutes which govern parking brakes, and air braked vehicles must have them. The USA, Asia and Europe have accepted the spring brake for parking brake requirements only and have not yet specified it for any emergency brake requirements. Since something is better than nothing, the trucking industry uses their legal parking brake for emergency situations because nothing else exists for them to use, if the service brakes fail when a vehicle is in motion. However, because of the limited stopping power of spring brakes, and since they are usually not used on all braked wheels, they are not effective replacements for the service brakes which are designed to stop a vehicle in motion.
The Bendix Corporation of Southfield, Michigan sells two models of air brakes, identified as DD-3 and SD-3 safety actuators, with mechanically locked push rods. The Bendix safety actuators include a number of rollers wedged by a spring between the push rod and an inclined collar ramp. During normal running conditions, that is when the spring brake line is pressurized, air pressure in the spring brake line pushes a piston against the rollers to move the rollers up the ramp so that the rollers move away from the push rod. When the spring brake line is vented, such as when parked, the piston returns allowing the rollers, under the influence of the spring, to be wedged against the collar ramp and the push rod; this locks the push rod in place.
When using the Bendix safety actuator, the brakes do not release by normal means. That is, people in the trucking industry are used to having the parking brakes release when the air pressure comes up. However, with the Bendix safety actuators, one cannot always release the parking brake unless an application of the service brake is also made. Also, application of the Bendix safety actuators requires that the vehicle's service brakes bring the vehicle to a stop before their application. Therefore, the Bendix safety actuators are usable only for parking, not emergency purposes, if a vehicle's service brake is unable to stop the vehicle while moving. The Bendix actuator is complex, requires many precision machined parts and needs perfect seals. It is thus quite expensive. At present the only equipment of which applicant is aware using the DD-3 and SD-3 are GMC transit buses. These drawbacks may account for the limited acceptance the Bendix safety actuators have achieved.
International Transquip Industries, Inc. of Houston, Texas sells an air activated mechanically held brake under the trademark Mini-Max. The brake is used with a conventional relay emergency valve for emergency application. For parking, serrations on the push rod are engaged by the serrated end of a piston to lock the push rod in place. This occurs when air pressure in the emergency tank is lost. Because of the loads and the environment encountered during use, it is expected that the serrated members would need to be made of expensive metals to achieve long life for this combination service and parking brake.